I was able to run 8 miles just a week and a half ago and now I can barely run 4 miles?

A week and a half ago i was able to run 8 miles almost everyday and i could run 40-45 minutes of HIIT training which i would do sprints on the treadmill for. This last week however i have been extreamly tired on the treadmill, i can only run like 4 miles and then i am extreamly tired and i like cant go on where as before i could run so much longer. I also cannot run at the same pace, as it is to fast for me, i have to run at a slower pace. Also i have to take more walking breaks whereas before i would not have to take walking breaks at all. I am getting way more tired way more quickly when i run and i dont know why. I also can only sprint for like 10 minutes and then i am completely exhausted. I am very scared about what is going on and i really do not want to loose the current condition i am in because i worked really hard to get here. Does anyone know why this is happening or what i can do to prevent it from happening?? How do i get back to running my normal amount??

ps. I eat very healthy, a salad a day, always eggs in the mornings, apples and veggies, and i try and drink a lot of water throughout the day. PLease help i really want to be able to run without feeling completely dead. I did not take a week off of working out or anything either... please help